An ink jet recording head of an on-demand type includes a plurality of nozzle openings and pressure generation chambers in communication with the respective nozzle openings and is structured such that the pressure generation chambers can be expanded and contracted in accordance with a printing signal to thereby generate ink drops.
By the way, when attached to a recording medium such as paper, the ink drops can blur on the paper depending on the quality of the paper or can be contacted with some other member to thereby cause a rub between them and, therefore, the ink is prepared in such a manner that the solvent thereof can be volatilized and solidified as rapidly as possible. Due to this, when a printing operation is interrupted, the ink solvent in the nozzle opening is volatilized rapidly to thereby cause the nozzle opening to be clogged up. In view of this, there are taken measures to install caps on the nozzle openings to thereby prevent the volatilization of the ink solvent.
On the other hand, during the printing operation, since new ink is supplied to the nozzle openings, the nozzle openings are more difficult to be clogged up. However, some of the nozzle openings, for example, nozzle openings which are situated in the upper and lower end portions of the recording head, have few chances to jet out the ink drops and, therefore, they can be clogged up easily.
In order to solve this problem, there is proposed a so called flushing operation in which, at a stage when the printing operation has been continued for a given period of time, the recording head is made to retreat into capping means which is disposed in a non-printing area, a drive signal is applied to piezo-vibrators to thereby jet out the ink drops from all nozzle openings forcibly toward their respective caps. This flushing operation can truly solve the nozzle opening clogging-up problem but it is necessary that the printing operation is interrupted and the recording head is moved to the capping means, which lowers the printing speed extremely.
In order to reduce the frequency of such flushing operation as much as possible, there are proposed a large number of technologies (for example, Japanese Patent Publication No. Sho. 55-123476, Japanese Patent Publication No. Sho. 57-61576, U.S. Pat. No. 4,350,989) in which, during the printing operation, a minute drive signal incapable of jetting out the ink drops is applied to the piezo-vibrators respectively provided in a pressure generation chamber in communication with the nozzle openings to thereby vibrate menisci disposed adjacent to the nozzle openings, in order to prevent the nozzle openings from being clogging up.
According to these technologies, there is eliminated the need to move the recording head to the capping position that is necessary for the flushing operation, thereby preventing the lowering of the printing speed. However, in a recording head using ink which is hard to blur on the recording medium, such as ink which contains therein resin emulsion to thereby be able to promote the quick formation of membrane, if the menisci are unnecessarily caused to vibrate slightly, then the ink solvent existing in the neighborhood of the menisci becomes easy to volatilize, which not only increases the viscosity of the ink but also promotes the membrane formation, thereby causing the nozzle openings to be clogged up.
In order to solve such problem, as disclosed in U.S. Pat. No. 5,329,293, there is proposed a printer of an ink jet type in which, after ink drops for printing are jetted out, the menisci are slightly vibrated at given intervals, thereby being able to prevent an increase in the viscosity of the ink which is caused by the unnecessary slight vibrations of the menisci.
However, in the actual printing, there has not been proposed yet a technology which is sure to prevent the nozzle openings from being clogged up until a series of operations have been finished; that is, in the series of operations, there are included an operation to put the power supply into work, an operation to remove the recording head from the caps, an operation to carry out a predetermined printing operation, and an operation to seal off the recording head again after completion of a printing operation.